Flat plate and light guide plate

ABSTRACT

A flat plate made of a cycloolefin polymer comprising at least 30% by weight, based on the cyclopolefin polymer, of repeating units (A) having an alicyclic structure. The repeating units (A) comprises at least 30% by weight, based on the repeating units (A), of repeating units (A-i) having no norbornane structure. The cycloolefin polymer has a weight average molecular weight of 5,000 to 50,000. The flat plate is especially useful as a light guide plate.

BACKGROUND OF THE INVENTION

[0001] (1) Field of the Invention

[0002] This invention relates to a flat plate having excellent lighttransmittance, reduced moisture absorption and enhanced mechanicalstrength, and which is thin and becomes colored only to a negligibleextent.

[0003] This flat plate is useful as a light guide plate and a lightdiffusion panel.

[0004] (2) Description of the Related Art

[0005] Characteristics such as a high light transmittance, a reducedthickness and a reduced distortion due to moisture absorption arerequired for light guide plates used as a liquid crystal base plate anda light diffuser panel. To provide a light guide plate having therequired characteristics, for example, an injection-molded plate hasbeen proposed in Japanese Unexamined Patent Publication No. H7-118344,which is made of a cycloolefin polymer having a norbornane ringstructure, made by metathesis-catalyzed polymerization of8-methyl-8-methoxycarbonyltetracyclododeca-3-ene, and having anintrinsic viscosity of 0.2 to 1.5 dl/g imparted thereto for enhancingthe fluidity upon molding. However, this proposal has problems suchthat, (1) for enhancing the fluidity to provide a thin plate, when themolecular weight of the cycloolefin polymer is reduced, the mechanicalstrength of the plate is reduced, or the plate is cracked upon moldingor becomes brittle, and, when the molding temperature is elevated, themolded plate becomes colored, and further that, in contrast, (2) whenthe molecular weight of the cycloolefin polymer is increased enough forimproving the mechanical strength, the melt fluidity is decreased and athin plate is difficult to obtain.

DISCLOSURE OF THE INVENTION

[0006] In view of the foregoing, a primary object of the presentinvention is to provide a flat plate such as a light guide plate, whichhas excellent light transmittance, reduced moisture absorption andenhanced mechanical strength, and which can be thin and becomes coloredonly to a negligible extent when molding is conducted at a hightemperature.

[0007] In accordance with the present invention, there is provided aflat plate made of a cycloolefin polymer comprising at least 30% byweight, based on the cycloolefin polymer, of repeating units (A) havingan alicyclic structure; said repeating units (A) comprising at least 30%by weight, based on the repeating units (A), of repeating units (A-i)having no norbornane structure; and said cycloolefin polymer having aweight average molecular weight of 5,000 to 50,000.

[0008] In accordance with the present invention, there is furtherprovided a light guide plate made of the above-mentioned flat plate.

BRIEF DESCRIPTION OF THE DRAWING

[0009]FIG. 1 is a cross-sectional view of a back light unit of a flatpanel display of an information display apparatus, which display isprovided with a light guide plate of the present invention.

[0010]FIG. 2 is a cross-sectional view illustrating one example of acombination of a light guide plate with a light source constituting aback light unit.

[0011]FIG. 3 is a cross-sectional view illustrating another example of acombination of a light guide plate with a light source constituting aback light unit.

[0012]FIG. 4 is an enlarged view of part II in FIG. 3.

[0013]FIG. 5 is a perspective view illustrating a plane of a guideplate, through which a light comes out.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0014] The cycloolefin polymer constituting the flat plate of thepresent invention is characterized as comprising at least 30% by weight,based on the cycloolefin polymer, of repeating units (A) having analicyclic structure; at least 30% by weight of said repeating units (A)being repeating units (A-i) having no norbornane

[0015] The alicyclic structure in the repeating units (A) may becontained in any of the backbone chain and/or the side chains. In thecase where especially high mechanical strength and transparency arerequired, the alicyclic structure is preferably contained in thebackbone chain. The alicyclic structure may be either saturated orunsaturated, but a saturated alicyclic structure is preferable in viewof high mechanical strength. As examples of the alicyclic structure,there can be mentioned cycloalkane and cycloalkene structures. Of these,a cycloalkane structure is preferable. The number of carbon atoms in thealicyclic structure can be suitably varied depending upon the intendeduse of the flat plate, but is usually in the range of 4 to 30,preferably 5 to 20 and more preferably 5 to 15 in view of enhancedmechanical strength.

[0016] The cycloolefin polymer may have either one kind of the repeatingunits (A) with an alicyclic structure or at least two kind of therepeating units with an alicyclic structure in the molecule. The contentof the repeating units (A) with an alicyclic structure in thecycloolefin polymer is suitably determined within the range of 30 to100% by weight, preferably from 50 to 100% by weight, more preferably 70to 100% by weight and most preferably 100% by weight, based on thecycloolefin polymer. If the content of the repeating units (A) with analicyclic structure in the cycloolefin polymer is too small, themechanical strength and light transmittance are poor.

[0017] The repeating units (A-i) with an alicyclic structure having nonorbornane structure is not particularly limited, but includes, forexample, those which are represented by the following formula (1):

[0018] wherein R¹ through R²⁰ are independently selected from a hydrogenatom, hydrocarbon, hydroxyl, ester, alkoxy, cyano, imido and silylgroups, a halogen atom, and a hydrocarbon group having a functionalgroup as substituent selected from a hydroxyl, ester, alkoxy, cyano,imido and sylyl groups; R⁸ and R¹⁰, or R⁹ and R¹¹ may form together anunsaturated bond; or R⁸ and R⁹, or R¹⁰ and R¹¹ may form together analkylidene group; a is 0 or 1, b is 0 or 1 and c is 0, 1 or 2; and

represents a carbon-carbon single or double bond.

[0019] More specifically, in formula (1), R¹ through R²⁰ areindependently selected from a hydrogen atom, hydrocarbon, hydroxyl,ester, alkoxy, cyano, imido and silyl groups, a halogen atom, and ahydrocarbon group having a functional group as substituent selected froma hydroxyl, ester, alkoxy, cyano, imido and silyl groups. R¹ through R²⁰are preferably selected from a hydrogen atom and hydrogen bond, and morepreferably a hydrogen atom. The carbon number in the hydrocarbon groupis usually in the range of 1 to 20, preferably 1 to 10 and morepreferably 1 to 6. The hydrocarbon group includes, for example, an alkylgroup and an alkenyl group, preferably an alkyl group. Preferably thealkyl group has 1 to 6 carbon atoms. The hydrocarbon group having asubstituent selected from the specified functional groups, includes, forexample, an alkyl group having 1 to 20 carbon atoms, preferably 1 to 10carbon atoms and more preferably 1 to 6 carbon atoms. The halogen atomincludes, for example, fluorine, chlorine, bromine and iodine atoms.

[0020] In formula (1), R⁸ and R¹⁰, or R⁹ and R¹¹ may form together anunsaturated bond; or R⁸ and R⁹, or R¹⁰ and R¹¹ may form together analkylidene group. The alkylidene group has usually 1 to 20 carbon atoms,preferably 1 to 10 carbon atoms and more preferably 1 to 6 carbon atoms.

[0021] In formula (1), a is 0 or 1, and preferably 0. b is 0 or 1, andpreferably 1. c is 0, 1 or 2, preferably 1.

represents a carbon-carbon single or double bond. In view of the lighttransmittance, the proportion of the carbon-carbon single bond to thetotal of the carbon-carbon single and double bonds is at least 95%,preferably at least 98% and more preferably at least 99%.

[0022] Of the repeating units (A-i) with an alicyclic structure havingno norbornane structure, represented by the formula (1), those which arerepresented by the following formula (2) are preferable, and that whichis represented by the following formula (3) is more preferable.

[0023] wherein definition and specific, examples of R²¹ through R³⁴ arethe same as R¹ through R²⁰in formula (1). In formula (2) R²⁵ and R²⁷, orR²⁶ and R²⁸ may form together an unsaturated bond; or R²⁵ and R²⁶, orR²⁷ and R²⁸ may form together an alkylidene group. The alkylidene grouphas usually 1 to 20 carbon atoms, preferably 1 to 10 carbon atoms andmore preferably 1 to 6 carbon atoms. In formula (2), d is 0 or 1, andpreferably 0.

represents a carbon-carbon single or double bond. In view of the lighttransmittance, the proportion of the carbon-carbon single bond to thetotal of the carbon-carbon single and double bonds is at least 95%,preferably at least 98% and more preferably at least 99%.

[0024] wherein

represents a carbon-carbon single or double bond. In view of the lighttransmittance, the proportion of the carbon-carbon single bond is atleast 95%, preferably at least 98% and more preferably at least 99%.

[0025] As specific examples of the repeating units (A-i) having analicyclic structure having no norbornane structure, there can bementioned those which are derived from ethylnorbornene,dicyclopentadiene, dihydrodicyclopentadiene, dimethyldicyclopentadiene,methyldicyclopentadiene, 5-ethylidene-bicyclo-[2.2.1]-hept-2-ene,5-ethyl-bicyclo[2.2.1]-hept-2-ene,tetracyclo-[7.4.0.0^(2,7).1^(10,13)]-tetradeca-11-ene andtetracyclo-[7.4.0.0^(2,7).1^(10,13)]-trideca-2,4,6,11-tetraene.

[0026] The repeating units (A-i) having an alicyclic structure having nonorbornane structure may be contained either alone or as a combinationof at least two thereof. The content of the repeating units (A-i) havingan alicyclic structure having no norbornane group in the entirerepeating units (A) having an alicyclic structure is in the range of 30to 100% by weight, preferably 50 to 100% by weight, more preferably 70to 100% by weight and most preferably 80 to 100% by weight. If thecontent of the repeating units (A-i) having an alicyclic structurehaving no norbornane group in the entire repeating units (A) having analicyclic structure is too small, the mechanical strength is poor,cracks are liable to occur at the molding step, and the shaped articleis brittle, namely, a thin shaped article is difficult to obtain. Incontrast, if the content of the repeating units (A-i) having analicyclic structure having no norbornane group in the entire repeatingunits (A) having an alicyclic structure is large, a high mechanicalstrength can be obtained even though the molecular weight of thecycloolefin polymer is reduced to improve the flowability.

[0027] Among the repeating units (A) having an alicyclic structure,repeating units other than the repeating units (A-i) having nonorbornane structure, namely, repeating units (A-ii) having a norbornanestructure, are not particularly limited, and, for example, include thosewhich are represented by the formula (4):

[0028] wherein definition and specific examples of R³⁵ through R⁴⁸ arethe same as R¹ in formula (1). R³⁸ and R⁴⁰, or R³⁹ and R⁴¹ may formtogether an unsaturated bond; or R³⁸ and R³⁹, or R⁴⁰ and R⁴¹ may formtogether an alkylidene group. The alkylidene group has usually 1 to 20carbon atoms, preferably 1 to 10 carbon atoms and more preferably 1 to 6carbon atoms. In formula (4), e is an integer of 1 to 3, and preferably1.

represents a carbon-carbon single or double bond. In view of the lighttransmittance, the proportion of the carbon-carbon single bond to thetotal of the carbon-carbon single and double bonds is at least 95%,preferably at least 98% and more preferably at least 99%.

[0029] Among the repeating units (A-ii) having an alicyclic structurehaving a norbornane structure, those which are represented by theformula (5) are preferable, and those which are represented by theformula (6) are preferable

[0030] wherein definition and specific examples of R⁴⁹ through R⁶² arethe same as R¹ in formula (1). In formula (5), R⁵² and R⁵⁴, or R⁵³ andR⁵⁵ may form together an unsaturated bond; or R⁵² and R⁵³, or R⁵⁴ andR⁵⁵ may form together an alkylidene group. The alkylidene group hasusually 1 to 20 carbon atoms, preferably 1 to 10 carbon atoms and morepreferably 1 to 6 carbon atoms. In formula (5),

represents a carbon-carbon single or double bond. In view of the lighttransmittance, the proportion of the carbon-carbon single bond to thetotal of the carbon-carbon single and double bonds is at least 95%,preferably at least 98% and more preferably at least 99%.

[0031] wherein definition and specific examples of R⁶³ through R⁶⁶ arethe same as R¹ in formula (1). In formula (6), R⁶³ and R⁶⁵, or R⁶⁴ andR⁶⁶ may form together an unsaturated bond; or R⁶³ and R⁶⁴, or R⁶⁵ andR⁶⁶ may form together an alkylidene group. The alkylidene group hasusually 1 to 20 carbon atoms, preferably 1 to 10 carbon atoms and morepreferably 1 to 6 carbon atoms. In formula (6),

represents a carbon-carbon single or double bond. In view of the lighttransmittance, the proportion of the carbon-carbon single bond to thetotal of the carbon-carbon single and double bonds is at least 95%,preferably at least 98% and more preferably at least 99%.

[0032] As specific examples of the repeating units (A-ii) having analicyclic structure having a norbornane structure, there can bementioned repeating units derived from8-ethyl-tetracyclo-[4.4.0.1^(2,5).1 ^(7,10)]-dodeca-3-ene,8-ethylidene-tetracyclo-[4.4.0.1^(2,5).1 ^(7,10)]-dodeca-3-ene,8-methyl-tetracyclo-[4.4.0.1^(2,5).1 ^(7,10)]-dodeca-3-ene, andtetracyclo[4.4.0.1^(2,5).1 ^(7,10)]-dodeca-3-ene.

[0033] The cycloolefin polymer may contain either one kind of therepeating units (A-ii) with an alicyclic structure having a norbornanestructure or a combination of at least two kinds thereof. The content ofthe repeating units (A-ii) with an alicyclic structure having anorbornane structure is the remainder of the total repeating units (A)with an alicyclic structure, other than the repeating units (A-i) withan alicyclic structure having no norbornane structure, namely, 0 to 70%by weight, preferably 0 to 50% by weight, more preferably 0 to 30% byweight and most preferably 0 to 20% by weight, based on the totalrepeating units (A) with an alicyclic structure.

[0034] The remainder of the repeating units, other than the repeatingunits (A) with an alicyclic structure, in the cycloolefin polymer isrepeating units (B) having no alicyclic structure. The repeating units(B) are not particularly limited, but usually include chain-likerepeating units. The chainlike repeating units include, for example,those which are represented by the following formula (7).

[0035] wherein R⁶⁷ through R⁷⁰ are the same as R¹ as defined in formula(1), and are preferably a hydrogen atom.

[0036] As specific examples of the chain-like repeating units, there canbe mentioned repeating units derived from α-olefins such as ethylene,propylene, 1-butene and 1-pentene, aromatic vinyls such as styrene andα-methylstyrene, cycloolefins such as cyclobutene, cyclopentene andcyclohexene, and non-conjugated dienes such as 1,4-hexadiene.

[0037] The molecular weight of the cycloolefin polymer as measured bygel permeation chromatography (GPC) using cyclohexane as the solvent (orusing toluene when the polymer is insoluble in cyclohexane) andexpressed in terms of a weight average molecular weight of polyisopreneis usually in the range of 5,000 to 50,000, preferably 7,000 to 45,000and more preferably 10,000 to 40.000. When the weight average molecularweight of the polymer is too small, the mechanical strength is poor. Incontrast, when the weight average molecular weight of the polymer is toolarge, the melt fluidity is poor.

[0038] The cycloolefin polymer used in the present invention preferablyhas a temperature of at least 280° C., preferably at least 360° C. andmore preferably at least 400° C., at which the loss of polymer weightupon heating reaches 5% by weight. In the case where the weight losstemperature upon heating is too low, when the injection molding isconducted at a high temperature in order to enhance the melt fluidity,the polymer is apt to be degraded. The resulting shaped plate hascontained therein bubbles of gas produced by the degradation of polymer,which lead to occurrence of silver streaks. When the weight losstemperature upon heating is not higher than 280° C., the undesirabledegradation of polymer does not occur even when the polymer is heated toa high temperature to enhance the melt fluidity, and thus, theproduction of bubbles and occurrence of silver streaks can be avoided.

[0039] The cycloolefin polymer used in the present invention has a meltviscosity of usually in the range of 1×10² to 1×10⁵ poise and preferably1×10² to 1×10³ poise as measured at a temperature of 260° C. If the meltviscosity is too large, when the polymer is injection-molded for theproduction of a large-size plate, short-shots occur and a light guideplate having a light reflecting plane having a minute pattern with ahigh precision cannot be obtained. The resulting light guide plate has apoor function of emitting light uniformly over the entire major surfacethereof. When the melt viscosity is in the above range, suchdisadvantage does not arise and thus a light guide plate with anenhanced processability is obtained.

[0040] The cycloolefin polymer used in the present invention may containa fraction having a molecular weight of not larger than 1,000 asmeasured by the above-mentioned GPC. The content of thelow-molecular-weight fraction is not particularly limited, but thecontent thereof is usually not larger than 20% by weight, preferably notlarger than 10% by weight, more preferably not larger than 5% by weightand most preferably not larger than 3% by weight. When the content ofthe low-molecular-weight fraction is not larger than 20% by weight, themechanical strength is enhanced and the balance between the mechanicalstrength and the moldability is good. The lower limit of the content: ofthe fraction with a molecular weight of not larger than 1,000 is notparticularly limited, but preferably the content of thelow-molecular-weight fraction is at least 0.2% by weight.

[0041] Production of Cycloolefin Polymer

[0042] The cycloolefin polymer used in the present invention can beproduced by the conventional process. For example, a process can beadopted in which an olefin monomer having an alicyclic structure ispolymerized in the presence of a metathesis catalyst system, and, ifdesired, the thus-obtained polymer is subjected to hydrogenation.

[0043] The olefin monomer (a) having an alicyclic structure preferablyincludes olefin monomers capable of forming repeating units having nonorbornane structure. As examples of the such olefin monomers (a), therecan be mentioned olefin monomers (a-i) represented by the followingformula (8), preferably those which are represented by the followingformula (9) and more preferably those which are represented by thefollowing formula (10). It is described in, for example, Japanese PatentNo. 2534086 and Japanese Examined Patent Publication No. H7-121981 thatthe olefin monomers (a-i) forms repeating units (A-i) when they arepolymerized in the presence of a metathesis catalyst.

[0044] wherein R¹ to R²⁰, and a, b and c are the same as defined withregard to formula (1).

[0045] wherein R²¹ to R³⁴ and d are the same as defined with regard toformula (2).

[0046] The olefin monomers (a-i) represented by the formulae (8), (9)and (10) may be used either alone or as a combination of at least twothereof. The proportion of these monomers (a-i) to the cycloolefinmonomers (a) having an alicyclic structure is in the range of 30 to 100%by weight, preferably 50 to 100% by weight, more preferably 70 to 100%by weight, especially preferably 80 to 100% by weight and mostpreferably 100% by weight.

[0047] The remainder of the cycloolefin monomers (a) having an alicyclicstructure, other than the olefin monomers (a-i) represented by theformulae (8), (9) and (10), includes alicyclic olefin monomersrepresented by the following formula (11), preferably alicyclic olefinmonomers (a-ii) represented by the following formula (12) and morepreferably alicyclic olefin monomers represented by the followingformula (13).

[0048] wherein R³⁵ to R⁴⁸, and e are the same as defined with regard toformula (4).

[0049] wherein R⁴⁹ to R⁶² are the sane as defined with regard to formula(5).

[0050] wherein R⁶³ to R⁶⁶ are the same as defined with regard to formula(6).

[0051] These cycloolefin monomers (a) having an alicyclic structure maybe used either alone or as a combination of at least two thereof. Theproportion of the cycloolefin monomers (a) to the total monomers is inthe range of 30 to 100% by weight, preferably 50 to 100% by weight, morepreferably 70 to 100% by weight and most preferably 100% by weight.

[0052] The remainder of the total monomers, other than the cycloolefinmonomers (a) having an alicyclic structure, usually includes chainstructure monomers. As examples of the chain structure monomers, therecan be mentioned those which are represented by the following formula(14).

[0053] wherein R⁶⁷ to R⁷⁰ are the same as defined with regard to formula(7).

[0054] As specific examples of combinations of cycloolefin monomersused, the following combinations can be mentioned.

[0055] (1) DCP [monomer (a)]+ETD [monomer (b)],

[0056] (2) DCP [monomer (a)]+ENB [monomer (a)],

[0057] (3) DCP [monomer (a)]+TCD [monomer (b)]+ETD [monomer (b)],

[0058] (4) DCP [monomer (a)]+MTF [monomer (a)]+TCD [monomer (b)],

[0059] and (5) MTF [monomer (a)]+TCD [monomer (b)],

[0060] wherein DCP: Dicyclopentadiene

[0061] ETD: Ethyl-tetracyclo[4.4.0.1^(2,5).1 ^(7,10)]-dodeca-3-ene

[0062] ENB: Ethylnorbornene (ethyl-bicyclo[2.2.1]-hept-2-ene

[0063] TCD: Tetracyclo[4.4.0.1^(2,5).1 ^(7,10)]-dodeca-3-ene

[0064] MTF: Tetracyclo[7.4.0.0^(2,7).1^(10,13)]-trideca-2,4,6,11-tetraene

[0065] The metathesis catalyst used includes, for example, compounds ofa metal of group VI of the periodic table such as tungsten (W) ormolybdenum (Mo), and compounds of a metal of group IV of the periodictable such as titanium (Ti). Of these, tungsten (W) compounds andmolybdenum (Mo) compounds are preferable. Tungsten (W) compounds aremost preferable.

[0066] As specific examples of the tungsten (W) compounds, there can bementioned WBr₂, WBr₃, WBr₆, WCl₂, WCl₄, WCl₅, WCl₆, WF₂, WF₄, WF₆, WI₂,WI₄, WI₆, WOBr₄, WOCl₄, WOF₄, WO₂, H₂WO₄, NaWO₄, K₂WO₄, (NH₄)₂WO₄,CaWO₄, CuWO₄, MgWO₄, (CO)₅WC(OCH₃)(CH₃), (CO₅)WC(OC₂H₅)(CH₃) and(CO₅)WC(OC₂H₅)(C₄H₅). Of these, WBr₂, WBr₃, WBr₆, WCl₂, WCl₄, WCl₅,WCl₆, WF₂, WF₄, WF₆, WI₂, WI₄, WI₆, WOBr₄, WOCl₄ and WOF₄ arepreferable. WBr₂, WBr₃, WBr₆, WCl₂, WCl₄, WCl₅, WCl₆, WF₂, WF₄, WF₆,WI₂, WI₄ and WI₆ are more preferable.

[0067] These metathesis catalysts can be used alone or as a combinationof at least two thereof. The amount of the metathesis catalyst used isusually in the range of 0.1 to 1.5 parts by weight, 0.1 to 1.0 part byweight and more preferably 0.1 to 0.5 part by weight, based on 100 partsby weight of the total monomers.

[0068] In general a cocatalyst is used in combination with themetathesis catalyst for the metathesis polymerization, the cocatalystinclude, for example, organoaluminum compounds and organotin compounds.Of these, organoaluminum compounds are preferable. As specific examplesof the organoaluminum compounds, there can be mentionedtrialkylaluminums such as trimethylaluminum, triethylaluminum,tripropylaluminum, tributylaluminum and triisobutylaluminum; andalkylaluminum halides such as diethylaluminum chloride and ethylaluminumdichloride. Of these, triethylaluminum, triisobutylaluminum anddiethylaluminum chloride.

[0069] These cocatalysts may be used either alone or as a combination ofat least two thereof. The amount of the cocatalyst used is usually inthe range of 0.01 to 30 mole, preferably 0.1 to 20 mole and morepreferably 0.1 to 10 mole, per mole of the metathesis catalyst. When asuitable amount of the cocatalyst is used, the formation of gel isminimized, the polymerization activity is enhanced and ahigh-molecular-weight polymer is easily obtained.

[0070] A combination of a tungsten (W) compound as the metathesiscatalyst with an organoaluminum compound as the cocatalyst ispreferable. But, in order to produce a cycloolefin polymer comprisingrepeating units (A-i) having no norbornane structure, having a highmolecular weight(more specifically, a weight average molecular weight of5,000 to 50,000, preferably 7,000 to 45,000 and more preferably 10, 000to 40,000), and containing a reduced amount of a low-molecular-weightfraction (more specifically, usually not larger than 20% by weight,preferably not larger than 10% by weight, more preferably not largerthan 5% by weight and especially preferably not larger than 3% byweight, based on the weight of the cycloolefin polymer, of a fractionhaving a molecular weight of not larger than 1,000.), it is preferableto use a reaction adjuster in combination with the metathesis catalystand the cocatalyst.

[0071] The reaction adjusters include at least one compound selectedfrom polar compounds containing active hydrogen such as an alcohol andan amine, and polar compounds containing no active hydrogen such as anether, an ester, a ketone and a nitrile. The active hydrogen-containingpolar compounds have a function of preventing or minimizing theformation of gel and giving a high polymer with an enhanced molecularweight. Among such active hydrogen-containing polar compounds, analcohol is preferable. The polar compounds containing no active hydrogenhave a function of preventing or minimizing the formation of alow-molecular-weight fraction. Among such polar compounds having noactive hydrogen, an ether, an ester and a ketone are preferable. Aketone is especially preferable.

[0072] As specific examples of the alcohol, there can be mentionedsaturated alcohols such as methanol, ethanol, propanol, isopropanol,butanol, isobutanol, t-butanol, pentanol, isopentanol, hexanol andcyclohexanol; and unsaturated alcohols such as phenol and benzylalcohol. Of these, propanol, isopropanol, butanol and isobutanol arepreferable.

[0073] As specific examples of the ether, there can be mentioneddimethyl ether, diethyl ether, diisopropyl ether, dibutyl ether,ethylene glycol dibutyl ether and triethylene glycol dibutyl ether. Ofthese, diisopropyl ether and diethyl ether are preferable.

[0074] As specific examples of the ester, there can be mentioned methylformate, ethyl formate, methyl acetate, ethyl acetate, propyl acetate,isopropyl acetate, methyl benzoate, ethyl benzoate, propyl benzoate andisopropyl benzoate. Of these, methyl acetate and ethyl acetate arepreferable.

[0075] As specific examples of the ketone, there can be mentionedacetone, methyl ethyl ketone, diethyl ketone, methyl phenyl ketone anddiphenyl ketone. Of these, acetone and methyl ethyl ketone arepreferable.

[0076] As specific examples of the nitrile, there can be mentionedacetonitrile, benzonitrile and t-butyronitrile. Of these, benzonitrileand t-butyronitrile are preferable.

[0077] These reaction adjusters can be used either alone or incombination. A combination of a polar compound containing activehydrogen with a polar compound containing no active hydrogen ispreferable. Combinations of an alcohol with a ketone; an alcohol with anitrile; an alcohol with an ether; and an alcohol with an ester areespecially preferable. The amount of the reaction adjusters is usuallyin the range of 0.01 to 20 mole, 0.1 to 10 mole and more preferably 1 to5 mole, per mole of the metathesis catalyst.

[0078] The polymerization reaction is usually conducted in the presenceof a solvent. The solvent used includes, for example, aromatichydrocarbons such as benzene, toluene and xylene, aliphatic hydrocarbonssuch as n-pentane, hexane and heptane, alicyclic hydrocarbons such ascyclopentane, cyclohexane and cyclooctane. Of these, toluene,cyclohexane and cyclooctane are preferable. Toluene and cyclohexane areespecially preferable. These solvents can be used either alone or as acombination of at least two thereof. The amount of the solvent isusually in the range of 10 to 1,000 parts by weight, preferably 50 to700 parts by weight and more preferably 100 to 500 parts by weight,based on 100 parts by weight of the monomers.

[0079] The polymerization temperature is usually in the range of −10° C.to 200° C., preferably 0° C. to 100° C. and more preferably 10° C. to80° C. The polymerization time is usually in the range of 30 minutes to10 hours, preferably 1 hour to 7 hours and more preferably 2 hours to 5hours. When the polymerization temperature is too high, the amount of afraction having a molecular weight of not larger than 1,000 isincreased. In contrast, when the polymerization temperature is too low,the rate of polymerization is too small and the conversion does notincrease to a desired extent.

[0080] If desired, after the completion of polymerization, ahydrogenation catalyst can be added to the polymer to effecthydrogenation of the polymer. The hydrogenation catalyst used is notparticularly limited provided that it is capable of being used generallyfor hydrogenation of olefin compounds. Usually homogeneous andheterogeneous catalysts can be used.

[0081] As specific examples of the heterogeneous catalyst, there can bementioned nickel, palladium, platinum and solid catalysts comprised ofsuch a metal supported on a carrier such as carbon, silica, diatomaceousearth, alumina or titainium oxide; for example, nickel/silica,nickel/diatomaceous earth, nickel/alumina, palladium/carbon,palladium/silica, palladium/diatomaceous earth and palladium/alumina.

[0082] The homogeneous catalyst includes, for example, a combination ofa transition metal compound with an alkylaluminum compound or analkyllithium compound. As specific examples thereof, there can bementioned cobalt acetate/triethylaluminum, cobalt acetate/triisobutylaluminum, nickel acetate/triethylaluminum, nickelacetate/tri-isobutylaluminum, nickel acetylacetonato/triethylaluminum,nickel acetylacetonato/tri-isobutylaluminum, titanocenechoride/n-butyllithium, and zirconocene chloride/n-butyllithium.

[0083] These hydrogenation catalysts can be used either alone or as acombination of at least two thereof. The amount of the hydrogenationcatalyst is usually in the range of 0.01 to 100 parts by weight,preferably 0.1 to 50 parts by weight and more preferably 1 to 30 partsby weight, based on 100 parts by weight of the polymer.

[0084] The hydrogenation reaction is conducted usually under a hydrogenpressure of 1 to 150 kg/cm² at a temperature of 0 to 250° C., preferably20 to 200° C. for 1 hour to 20 hours.

[0085] After the hydrogenation reaction, the polymer is filtered toremove the hydrogenation catalyst, and then coagulated and dried to givethe cycloolefin polymer used in the present invention. In the case wherea homogeneous catalyst system is used as the hydrogenation catalyst,after the completion of hydrogenation, an alcohol or water is added todeactivate the catalyst and render the polymer insoluble in the solvent,and then the polymer is filtered, coagulated and dried to give thecycloolefin polymer used in the present invention.

[0086] Other Polymer Components

[0087] If desired, other polymer components can be incorporated with thecycloolefin polymer used in the present invention. The polymercomponents include rubber polymers and resins.

[0088] As specific examples of the rubber polymers, there can bementioned diene rubbers such as natural rubber, polybutadiene rubber,polyisoprene rubber, and acrylonitrile-butadiene copolymer rubber;styrene-butadiene copolymer rubber, styrene-isoprene copolymer rubber,and styrene-butadiene-isoprene terpolymer rubber; hydrogenation productsof diene rubbers; ethylene-α-olefin copolymer rubbers such asethylene-propylene copolymer rubber, saturated polyolefin rubbers suchas copolymers of propylene with other α-olefins, α-olefin-diene-typecopolymer rubbers such as ethylene-propylene-diene copolymer rubber,α-olefin-diene copolymer rubber, isobutylene-isoprene copolymer rubber,and isobutylene-diene copolymer rubber; special rubbers such as urethanerubber, silicone rubber, polyether rubber, acrylic rubber, propyleneoxide rubber, and ethylene acrylic rubber; thermoplastic elastomers suchas styrene-butadiene-styrene block copolymer rubber, andstyrene-isoprene-styrene block copolymer rubber; hydrogenatedthermoplastic elastomers; thermoplastic urethane elastomer;thermoplastic polyamide elastomer; and thermoplastic 1,2-polybutadieneelastomer.

[0089] As specific examples of the resins to be incorporated with thecycloolefin polymer, there can be mentioned polyolefins such aslow-density polyethylene, high-density polyethylene, straight chainlow-density polyethylene, ultra-low-density polyethylene, polypropylene,syndiotactic polypropylene, polybutene and polypentene; polyesters suchas polyethylene tereplithalate and polybutylene terephthalate;polyamides such as nylon 6 and nylon 66; ethylene-ethyl acrylatecopolymer, ethylene-vinyl acetate copolymer, polystyrene, syndiotacticpolystyrene, polyphenylene sulfide, polyphenylene ether, andpolycarbonate.

[0090] These polymer components can be used either alone or as acombination of at least two thereof. The amount of the polymer componentis usually not larger than 100 parts by weight, preferably not largerthan 70 parts by weight, more preferably not larger than 50 parts byweight and most preferably not larger than 30 parts by weight, based on100 parts by weight of the cycloolefin polymer.

[0091] Additives

[0092] Additives can be incorporated in the cycloolefin polymer used inthe present invention, according to the need. The additives used are notparticularly limited provided that they are generally used in resinindustries, and include, for example, antioxidants, light stabilizers,ultraviolet absorbers, lubricants, plasticizers, antistatic agents andlight diffusing agents.

[0093] The antioxidants include, for example, phenolic antioxidants,phosphorus-containing antioxidants and sulfur-containing antioxidants.Of these, phenolic antioxidants are preferable. Alkyl-substitutedphenolic antioxidants are especially preferable.

[0094] As the phenolic antioxidants, known phenolic antioxidants can beused and, as specific examples thereof, there can be mentioned acrylatecompounds described in Japanese Unexamined Patent Publication(hereinafter abbreviated to “JP-A”) S63-179953 and JP-A H1-168643, suchas 2-t-butyl-6-(3-t-butyl-2-hydroxy-5-methylbenzyl)-4-methylphenylacrylate, 2,4-di-t-amyl-6-(1-(3,5-di-t-amyl-2-hydroxyphenyl)ethyl)phenylacrylate; alkyl-substituted phenolic compounds such asoctadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl) propionate,2,2′-methylene-bis(4-methyl-6-t-bytylphenyl),1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane,1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydoxybenzyl)benzene,tetrakis(methylene-3-(3′,5′-di-t-butyl-4′-hydroxyphenyl)propionate)methane[namely, pentaerythtyryl-tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]], triethylene glycolbis(3-(3-t-butyl-4-hydroxy-5-methylphenyl) propionate); and triazinegroup-containing phenolic compounds such as6-(4-hydroxy-3,5-di-t-butylanilino)-2,4-bis-octylthio -1,3,5-triazine,and 6-(4-hydroxy-3,5-dimethylanilino)-2,4-bis-octylthio-1,3,5-triazine.

[0095] The phosphorus-containing antioxidants are not particularlylimited provided that they are generally used in resin industries. Asspecific examples of the phosphorus-containing antioxidants, there canbe mentioned monophosphite compounds such as triphenyl phosphite,diphenylisodecyl phosphite, phenylisodecyl phosphite, tris(nonylphenyl)phosphite, tris(dinonylphenyl) phosphite, tris(2,4-di-t-butylphenyl)phosphite, tris(2-t-butyl-4-methylphenyl) phosphite,tris(cyclohexylphenyl) phosphite,2,2-methylene-bis(4,6-di-t-butylphenyl)octyl phosphite,9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide,10-(3,5-di-t-butyl-4-hydroxybenzyl)-9,10-dihydro-9-oxa-10-phosphaphnanthrene-10-oxide,and 10-decyloxy-9,10-dihydro-9-oxa-10-phosphaphenanthrene; anddiphosphite compounds such as4,4′-butylidene-bis(3-methyl-6-t-butylphenyl-di-tridecyl phosphite),4,4′-isopropylidene-bis(phenyl-di-alkyl(C₁₂-C₁₅) phosphite),4,4′-isopropylidene-bis(diphenyl-mono-alkyl(C₁₂-C₁₅) phosphite),1,1,3-tris-(2-methyl-4-di-tridecyl phosphite -5-t-butylphenyl)butane,tetrakis(2,4-di-t-butylphenyl)-4,4′-biphenylene diphosphite, cyclicneopentanetetrayl bis(2,4-di-t-butylphenyl phosphite), cyclicneopentanetetrayl bis-(2,4-dimethylphenyl phosphate), and cyclicneopentanetetrayl bis(2,6-di-t-butylphenyl phosphate). Of these,monophosphite compounds are preferable. Tris(nonylphenyl) phosphate,tris(dinonylphenyl) phosphate and tris(2,4-di-t-butylphenyl) phosphiteare especially preferable.

[0096] As specific examples of the sulfur-containing antioxidants, therecan be mentioned dilauryl 3,3-dithiopropionate, dimyristil3,3-dithiopropionate, distearyl 3,3-dithiopropionate, laurylstearyl3,3-dithiopropionate,pentaerythritol-tetrakis-(β-lauryl-thio-propionate), and3,9-bis(2-dodecylthioethyl)-2,4,8,10-tetraoxaspiro[5,5]-undecane.

[0097] As preferable examples of the light stabilizer, there can bementioned hindered amine light stabilizers such as2,2,6,6-tetramethyl-4-piperidyl benzoate,bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate, bis(1,2,2,6,6-pentamethyl-4-piperidyl)-2-n-butyl malonate, and 4-(3-(3,5-di-t-butyl-4-hydroxyphenyl)propionyloxy)-1-(2-(3-(3,5-di-t-butyl-4-hydroxyphenyl)propionyloxy)ethyl)-2,2,6,6-tetramethyl-piperidine.

[0098] As specific examples of the ultraviolet absorbers, there can bementioned benzotriazole ultraviolet absorbers such as2-(2-hydroxy-5-methylphenyl)-2H-benzotriazole, 2-(3-t-butyl-2-hydroxy-5-methylphenyl)-5-chloro-2H-benzotriazole,2-(3,5-di-t-butyl-2-hydroxyphenyl)-5-chloro-2H-benzotriazole,2-(3,5-di-t-butyl-2-hydroxyphenyl)-2H-benzotriazole, 5-chloro-2-(3,5-di-t-butyl-2-hydroxyphenyl)-2H-benzotriazole, and2-(3,5-di-t-amyl-2-hydroxyphenyl)-2H-benzotriazole; benzoate ultravioletabsorbers such as 4-t-butylphenyl-2-hydroxy-benzoate,phenyl-2-hydroxybenzoate, 2,4-di-t-butylphenyl-3,5-di-t-butyl-4-hydroxybenzoate, hexadecyl-3,5-di-t-butyl-4-hydroxybenzoate,2-(2H-benzotriazol-2-yl)-4-methyl-6-(3,4,5,6-tetrahydrophthalymidylmethyl)phenol,2-(2-hydroxy -5-t-octylphenyl)-2H-benzotriazole and2-(2-hydroxy-4-octylphenyl)-2H-benzotriazole; benzophenone ultravioletabsorbers such as 2,4-dihydroxybenzophenone,2-hydroxy-4-methoxybenzophenone,2-hydroxy-4-methoxybenzophenone-5-sulfonic acid trihydrate,2-hydroxy-4-octyloxybenzophenone, 4-dodecyloxy-2-hydroxybenzophenone,4-benzyloxy-2-hydroxybenzophenone, 2,2′,4,4′-tetrahydroxybenzophenoneand 2,2′-dihydroxy-4,4′-dimethoxybenzophenone; acrylate ultravioletabsorbers such as ethyl-2-cyano-3,3-diphenyl acrylate and2′-ethylhexyl-2-cyano-3,3-diphenyl acrylate; and nickel complexultraviolet absorbers such as[2,2′-thiobis(4-t-octylphenolate)]-2-ethylhexylamine nickel.

[0099] As the lubricants, finely divided inorganic particles aregenerally used. As examples of the inorganic particles, there can bementioned particles of oxide, hydroxide, sulfide, nitride, halide,carbonate, sulfate, acetate, phosphate, phosphite, organic carboxylate,silicate, titanate, borate of an element selected from those of group 1,group 2, group 4, group 6, group 7, groups 8 to 10, group 11, group 12,group 13 and group 14; and hydrate compounds thereof; compositecompounds comprising these compounds as the main ingredient; and naturalmineral.

[0100] As specific examples of the plasticizer, there can be mentionedphosphoric acid triester plasticizers such as tricresyl phosphate,trixylyl phosphate, triphenyl phosphate, triethylphenyl phosphate,diphenylcresyl phosphate, monophenyldicresyl phosphate,dicresylmonoxylenyl phosphate, arylalkyl phosphate, diphenylmonoxylenylphosphate, monophenyldixylenyl phosphate, tributyl phosphate, triethylphosphate, trichloroethyl phosphate, trioctyl phosphate andtris(isopropylphenyl) phosphate; phthalic acid ester plasticizers suchas dimethyl phthalate, diethyl phthalate, dibutyl phthalate, diheptylphthalate, di-n-octyl phthalate, di-2-ethylhexyl phthalate, di-isononylphthalate, octyldecyl phthalate and butylbenzyl phthalate; aliphaticmonobasic acid ester plasticizers such as butyl oleate and glycerinemonooleic acid ester; aliphatic dibasic acid ester plasticizers such asdibutyl adipate; dihydric alcohol ester plasticizers; and oxy-acid esterplasticizers. Of these, phosphoric acid triester plasticizers arepreferable. Tricresyl phosphate and trixylyl phosphate are especiallypreferable.

[0101] Hydrocarbon polymers which are liquid at normal temperature andpredominantly have a C—C or C═C backbone structure can also be used as asoftening agent or plasticizer. Among the liquid hydrocarbon polymers,straight chain or branched chain liquid hydrocarbon polymers having nohydrocarbon ring in the backbone are preferably used. In view of theweather resistance, hydrocarbon polymers having no a C═C backbonestructure are preferable. The liquid hydrocarbon polymers used havepreferably a molecular weight of not larger than 10,000, more preferably200 to 8,000 and especially preferably 300 to 4,000.

[0102] As specific examples of the liquid hydrocarbon polymers, therecan be mentioned squalane (C₃₀H₆₂, MW:422.8), liquid paraffin (includingwhite oil, ISO VG10, ISO VG15, ISO VG32, ISO VG68, ISO VG100, VG8 andVG12, stipulated in JIS K2231), polyisobtene, and hydrogenatedpolybutadiene. Of these, squalane, liquid paraffin and polyisobutene arepreferable.

[0103] As specific examples of the antistatic agents, there can bementioned alkylsulfonate salts such as sodium alkylsulfonate andphosphonium alkylsulfonate, and glycerol stearate.

[0104] The above-recited additives can be used either alone or as acombination of at least two thereof. The amount of the additive issuitably determined within a range in which the object of the inventioncan be achieved.

[0105] Flat Plate and Light Guide Plate

[0106] By the term “flat plate” used herein, we mean a shaped articlehaving a flat shape wherein the ratio (L/T) of the minimum width L ofthe major surface to the maximum thickness T is at least 2. The minimumwidth L signifies length of the short side when the major surface isrectangular; diameter when the major surface is circle; and length ofthe minor axis when the major surface is oval. The maximum thickness Tsignifies the largest length as measured in the direction perpendicularto the major surface. The shape and size of the flat plate are notparticularly limited and are suitably determined depending upon theparticular use thereof. The shape can be any of square, rectangle,pentagonal or other polygonals, circle, ellipse, and other shapes. Thesize of the major surface can be varied in a broad range spanning from aminimum width of about 0.5 mm to a minimum width of about 100 cm orlonger. The thickness T of the flat plate is suitably determineddepending upon the particular use thereof, but is usually in the rangeof 0.5 to 10 mm, preferably 0.6 to 5.0 mm and more preferably 0.8 to 2.0mm.

[0107] Usually a shaped article has a phase difference occurring due tothe birefringence. The flat plate of the present invention usually has aphase difference not larger than 100 nm, preferably not larger than 70nm and more preferably not larger than 50 nm. When the phase differenceas measured in the direction of thickness is not larger than 100 nm, anoptical element such as a light guide plate having enhanced and uniformluminance can be obtained. The phase difference in the direction ofthickness is measured by the method hereinafter described.

[0108] A flat plate such as a light guide plate having such a reducedphase difference can be obtained, for example, by a method of annealinga shaped article to reduce the birefringence; a method of cutting toremove a part with a high birefringence of an injection-molded article,such as a part located in vicinity to the gate of a mold, whereby theremainder having a relatively low birefringence is selectively used; anda method of improving fluidity of the cycloolefin polymer to reduce theoccurrence of birefringence upon molding.

[0109] The flat plate of the present invention preferably has a highsurface precision, namely, the surface roughness as expressed as themaximum height Rmax is usually not larger than 5 μm, preferably notlarger than 1 μm and more preferably 0.5 μm. When the flat plate havingsuch a high surface precision is used as an optical element such as alight guide plate, uniformity and magnitude of luminance on theilluminating plane can be enhanced. The surface with such a reducedmaximum height Rmax can be obtained, for example, by a method ofpolishing the surface of plate; a method of molding a cycloolefinpolymer under selected conditions whereby defects such as burn mark canbe avoided and the surface precision are enhanced; and a method ofremoving foreign matter from as-produced polymer in thepolymer-producing step or from a molten polymer at the molding step, byusing, for example, a filter.

[0110] The flat plate of the present invention can be thin and has highlight transmittance, low moisture absorption, high mechanical strengthand enhanced discoloration resistance. Therefore, the flat plate issuitable for optical use, for example, a light guide plate provided in alight illuminating apparatus of an information-giving device.

[0111] By the term “light guide pate” used herein we mean a plate-formoptical element through which light from a light source transmits and isemitted uniformly from the major surface For example, a light guideplate provided in a plane light illuminating apparatus of aninformation-giving device has a function of allowing light to transmit,which is incident on the incident side end of the plate from afluorescent lamp, through the plate, and reflecting the light to adirection perpendicular to the major surface thereof and emitting thelight uniformly from the major surface (i.e., light illuminatingsurface).

[0112] Referring to FIG. 1, in a surface illuminating apparatus 10, alight guide plate 2 constitutes the surface illuminating apparatus (backlight unit) 10 in combination with a fluorescent lamp 4, reflector 3, areflecting sheet 6 and a diffusion panel 5. If desired, a condenserprism (not shown) for conversing light is provided on the diffusionpanel 5, and this assembly is combined with a liquid crystal, a colorfilter and other sheet-form elements to constitute a flat panel displayunit forming a display of, for example, a portable computer, a carnavigating system or a thin television picture tube.

[0113] The light emitted from the fluorescent lamp 4 is reflected by thereflector 3 and is incident on an incident side end of the light guideplate 2. Then the light is passed through the plate 2 and reflected byan under major surface (reflecting surface) of the plate. The reflectedlight transmits in a direction perpendicular to a top major surfaceforming an illuminating surface and emits upward from the illuminatingsurface. Then the light transmits through the diffusion panel 5 to adisplay of an information-giving device.

[0114] The light guide plate 2 may be of a flat shape, but preferablyhas a wedge-shaped cross-section which has a thickness graduallydecreasing from the light incident end to the opposite end. The lightguide plate 2 has a printed pattern on the reflecting face of the bottomthereof or has a plurality of grooves with V-shaped cross-section on thereflecting face of the bottom thereof (as illustrated in FIGS. 1 to 3),in order to reflect the light to the top illuminating surface. The lightguide plate of the present invention preferably has grooves withV-shaped cross-section the reflecting face of the bottom thereof. Thelight guide plate having a printed pattern on the reflecting face hasdisadvantages such that the efficiency of light reflection is relativelylow, and that two steps, i.e., a plate-shaping step and a printing step,are necessary for making the plate. In contrst, the light guide platemade of the cycloolefin polymer used in the, present invention hasminute grooves with high precision, because the cycloolefin polymerexhibits good capability of following minute grooves on the inner moldsurface. Thus, the light guide plate with minute grooves on thereflecting face can be made by one step including plate-shaping andgroove-forming. Further the plate exhibits a relatively high reflectionefficiency.

[0115] A plurality of minute V-shaped grooves extending in parallel andperpendicular to the direction of light transmission are formed on thereflecting face of the light guide plate, as illustrated in FIG. 1 and apartial enlarged view. The V-shaped grooves have pitches graduallydecreasing from the light incident side end to the opposite end(right-hand end in FIG. 1) so that the reflected light is emitteduniformly over the entire area of the top illuminating surface.

[0116] Referring to FIG. 2, a surface illuminating device 1, light isincident from a lamp 4 on the incident side end 2 a and is allowed totransmit through a light guide plate 2. The light is reflected on thereflecting face 2 c having formed thereon a plurality of grooves 22 withV-shaped cross-section. The light guide plate 2 has a thicknessgradually decreasing from the light incident side end 2 a to theopposite end 2 d, and thus, a wedge-shaped cross-section. The grooveshave pitches gradually decreasing from the light incident side end 2 ato the opposite end 2 d so that the light reflected by the reflectingface 2 c emits uniformly over the illuminating surface 2 b.

[0117] In the light guide plate illustrated in FIG. 1 and FIG. 2, themajor surface usually has an area of 10 to 2,500 cm², preferably 100 to2,500 cm², more preferably 200 to 2,500 cm², and especially preferably500 to 2,500 cm². The thickness is usually in the range of 0.5 mm to 5mm. Top angle of each V-shaped groove (shown as α in the partialenlarged view in FIG. 1) is usually 100 to 170° and preferably 120 to150°. The depth of the V-shaped grooves is usually 0.1 to 100 μm andpreferably 1 to 50 μm. The ratio of the minimum pitch of grooves formedat the right hand end to the maximum pitch of grooves formed at thelight incident side end is usually 1/20 to 1/2 and preferably 1/10 to1/3. The maximum pitch is usually 0.1 to 10 mm and preferably 0.2 to 5mm. The slanting angle of the light reflecting face, shown as θ in FIG.2, is 0.1 to 15°.

[0118] Referring to FIG. 3 and FIG. 4, a light guide plate 2 withwedge-shaped cross-section having a structure similar to those shown inFIG. 1 and FIG. 2 is illustrated. In this light guide plate, pitches P1between adjacent grooves 22 in vicinity of the light incident side end 2a is usually 50 to 10,000 μm, preferably 100 to 5,000 μm and morepreferably 500 to 2,000 μm, and pitches P1 between adjacent grooves invicinity of the opposite end 2 d is usually 10 to 1,000 μm, preferably30 to 500 μm and more preferably 50 to 100 μm. The ratio of P1 invicinity of the light incident side end 2 a to P1 in vicinity of theopposite end 2 d is preferably in the range of 10/1 to 3/1. The width P2of groove 22 is usually 10 μm to 5 mm, preferably 30 μm to 2 mm and morepreferably 50 μm to 1 mm. The depth H1 of groove 22 is usually 1 to 500μm, preferably 10 to 100 μm and more preferably 30 to 80 μm. The angle θof groove 22 is 70 to 150°, preferably 90 to 130° and more preferably100 to 120°.

[0119] As illustrated in FIG. 4, ridges 24 are formed at both sides ofeach groove 22 having a height H2 of usually 10 to 150 μm, preferably 20to 120 μm and more preferably 25 to 100 μm. The ratio of the height H2of ridge 24 to the depth H1 of V-shaped groove 22 is in the range of 10to 500%, preferably 20 to 300% and more preferably 30 to 200%. The widthP3 of ridge 24 is usually 10 to 1,000 μm, preferably 50 to 600 μm andmore preferably 100 to 400 μm. The ratio of the cross-sectional area S22of each groove 22 to the cross-sectional area S24 of each ridge 24 isusually in the range of 10/1 to 10/100, preferably 10/2 to 10/50 andmore preferably 10/5 to 10/20.

[0120] As illustrated in FIG. 5, minute grooves 11 can be formed on anilluminating surface of the light guide plate 1 of the present inventionof a light emitting device 1. By forming the minute grooves 11 on theilluminating surface of plate, a condenser panel can be omitted. Theminute grooves are formed simultaneously with the molding of the plate.Thus, assembly of a plane light emitting apparatus can be conducted in asimple manner with use of a reduced number of parts.

[0121] The flat plate including the light guide plate of the presentinvention can be made from a composition comprising the above-mentionedcycloolefin polymer and other ingredients. The shaping method is notparticularly limited, and may be conventional. For example, injectionmolding, inflation shaping, compression molding, extrusion shaping, castshaping, and continuous extrusion shaping can be conducted. Of these, aninjection molding method is preferable because of good flowability ofpolymer and processability. The injection molding conditions aresuitably chosen depending upon the particular shape, size andcomposition of plate. The molding polymer temperature is usually in therange of 100 to 400° C., preferably 150 to 350° C. and more preferably200 to 300° C. in view of balanced flowability and discolorationresistance.

[0122] It is not preferable, however, that the light guide plate is madeof a flat plate or sheet prepared by drawing, because the polymer isoriented in the direction of drawing, and thus, the phase differenceoccurring due to birefringence as measured in the thickness direction ofthe plate is undesirably large.

[0123] The procedure for making a light guide plate having a shape asillustrated in FIGS. 1 to 5 can also be applied to cycloolefin polymersother than the above-mentioned cycloolefin polymer.

[0124] The light guide plate of the present invention with V-shapedgrooves and further ridges on the light reflecting face, made of thespecific cycloolefin polymer, is beneficial in that the melt-flowabilityof polymer is high, and thus, grooves and ridges with high precision canbe formed simultaneously with molding of plate. The cycloolefin polymerhas a high mechanical strength, and thus, when the molded plate isreleased from a mold at the step of molding, cracks and cutouts do notoccur and the plate is not distorted. The light guide plate exhibitsenhanced and uniform luminance at the illuminating surface thereof.

[0125] The invention will now be specifically described by the followingexamples. Parts and % in the examples are by weight unless otherwisespecified. Properties of polymer and polymer plate were evaluated by thefollowing methods.

[0126] (1) Weight Average Molecular Weight

[0127] The weight average molecular weight was determined by gelpermeation chromatography (GPC) using cyclohexane (or toluene when thepolymer is not soluble in cyclohexane) as a solvent, and was expressedas weight average molecular weight of polyisoprene.

[0128] (2) Low-Molecular-Weight Fraction

[0129] The content of a fraction having a weight average molecularweight of not larger than 1,000 was calculated from the results ofdetermination of the weight average molecular weight described in thepreceding paragraph (1).

[0130] (3) Hydrogenation Percentage

[0131] The hydrogenation percentage was determined by ¹H-NMR.

[0132] (4) Glass Transition Temperature

[0133] The glass transition temperature was determined by DSC method.

[0134] (5) Mechanical Strength

[0135] (A) Crack Occurrence at Molding Step

[0136] Formation of cracks and cutouts in a molded plate were observedwhen the plate was released from a mold.

[0137] The test was conducted on ten plate specimens and the testresults were evaluated according to the following four ratings.

[0138] a: Cracks or cutouts did not occur at all.

[0139] b: Cracks or cutouts occurred in one to three plates among theten plates.

[0140] c: Cracks or cutouts occurred in four to six plates among the tenplates.

[0141] d: Cracks or cutouts occurred in seven or more plates among tenplates.

[0142] The above-test was repeated many times and the percentage ofcrack or cutout formation was determined.

[0143] (B) Crack Occurrence at Drop-Weight Test

[0144] A missile-shaped weight (50 g) having a diameter of ¾ inch wasgravity-dropped from a height of 1 m onto a plate specimen. Thedrop-weight test was conducted on ten plate specimens and the testresults were evaluated according to the following four ratings.

[0145] a: Cracks or fractures did not occur at all.

[0146] b: Cracks or fractures occurred in one to three plates among theten plates.

[0147] c: Cracks or fractures occurred in four to six plates among theten plates.

[0148] d: Cracks or fractures occurred in all of the ten plates.

[0149] The above-test was repeated many times and the percentage ofcrack or fracture occurrence was determined.

[0150] (6) Distortion Due to Moisture Absorption

[0151] A flat plate specimen was allowed to stand for one week at atemperature of 23° C. and a relative humidity of 60% in a thermostaticchamber, and warpage due to moisture absorption was observed. Theresults were expressed according to the following ratings.

[0152] a: No warpage did not occur at all.

[0153] b. Warpage occurred.

[0154] (7) ΔYI Value

[0155] ΔYI value was measured by a color difference meter (SM colorcomputer; made by Suga Tester K.K.). The test results were expressedaccording to the following four ratings.

[0156] a: ΔYI value was below 0.5.

[0157] b: ΔYI value was at least 0.5 but below 0.8.

[0158] C: ΔYI value was at least 0.8 but below 1.1.

[0159] d: ΔYI value was at least 1.1.

[0160] (8) Occurrence of Emission Points and Dark Points

[0161] Occurrence of emission points and dark points on an illuminatingsurface of a light guide plate was visually observed by the naked eye.

[0162] (9) Surface Roughness Rmax

[0163] Surface roughness Rmax was expressed by maximum height (unit: μm)as measured by using a stylus surface roughness tester with a resolvingpower of 0.02 μm.

[0164] (10) Phase Difference in Thickness Direction

[0165] Birefringence of a light guide plate was measured by abirefringence measuring apparatus (made by Oak Co., ADR-100XY). Themeasurement was conducted at voluntarily chosen three points, on themajor surface, which are 1 cm apart from the incident side end, and anaverage value (unit: μm) was calculated.

[0166] Preparation of Polymer

REFERENCE EXAMPLE 1

[0167] A flask having a volume of 1 liter was flashed with nitrogen andwas charged with 5 g of dicyclopentadiene monomer and 120 g ofcyclohexane. 0.57 m-mol of tri-isobutylaluminum (iBu₃Al) aspolymerization catalyst, 0.57 m-mol of isobutyl alcohol and 0.189 m-molof acetone as reaction adjuster, and 4.55 m-mol of 1-hexene as molecularweight modifier were added. To the content, 0.076 m-mol of tungstenhexachloride was added and the mixture was maintained at 60° C. for 5minutes with stirring. Then 45 g of dicyclopentadiene monomer and amixture of 0.114 m-mol of tungsten hexachloride with cyclohexane werecontinuously dropwise added to the reaction mixture while the reactionmixture was maintained at 60° C. After completion of the dropwiseaddition, the reaction mixture was further stirred for 30 minutes tocomplete a ring-opening polymerization.

[0168] The thus-obtained polymer had a weight average molecular weight(Mw) of 16,000. Analysis of the polymer solution by gas chromatographyrevealed that peak of unreacted monomer was not detected, and thus, theconversion was 100%. The content of repeating units having an alicyclicstructure with two rings having no norbornane structure was 100% byweight.

[0169] The polymer solution was placed in an autoclave having a volumeof 1 liter, and 160 g of cyclohexane was added thereto. Then 2.5 g of anickel catalyst supported on a diatomaceous earth as hydrogenationcatalyst and 2.5 g of activated alumina having a surface area of 320cm²/g, a pore volume of 0.8 cm³/g and an average particle diameter of 15μm (Neobead D powder, supplied by Mizusawa Kagaku K.K.) were added. Theautoclave was flashed with hydrogen and the inner pressure was elevatedat about 10 kg/cm² and the temperature was elevated to 160° C. whilebeing stirred. When the temperature was stabilized, the hydrogenpressure was maintained at 40 kg/cm² and a reaction was conducted for 8hours while hydrogen consumed during the reaction was supplemented.

[0170] After completion of the hydrogenation reaction, the hydrogenationcatalyst and the activated alumina were removed by filtration, and 0.1part by weight, based on 100 parts by weight of the hydrgenated polymer,ofpentaerythrityl-tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]as antioxidant was added. The mixture was dried at 380° C. under areduced pressure to remove the solvent. Then the molten polymer waspelletized in a nitrogen atmosphere by using an extruder to give apelletized polymer [polymer (A)]. Polymer (A) had a weight averagemolecular weight (Mw) of 33,000, and contained 0.5% by weight of afraction with a molecular weight of not larger than 1,000. Thehydrogenation percentage of polymer (A) was 99.9%. The glass transitiontemperature was 97° C.

REFERENCE EXAMPLE 2

[0171] A polymer (B) was prepared by ring-opening polymerization andhydrogenated by the same procedures as mentioned in Reference Example 1except that acetone as reaction adjuster was not used and the reactiontemperature was changed to 70° C. Polymer (B) had a weight averagemolecular weight (Mw) of 32,400, and contained 10.0% by weight of afraction with a molecular weight of not larger than 1,000. Thehydrogenation percentage of polymer (B) was 99.9%. The glass transitiontemperature was 97° C. The content of repeating units having analicyclic structure with two rings having no norbornane structure was100% by weight.

REFERENCE EXAMPLE 3

[0172] A polymer (C) was prepared by ring-opening polymerization andhydrogenated by the same procedures as mentioned in Reference Example 1except that a monomer mixture composed of dicyclopentadiene and6-methyl-1,4:5,8-dimethano-1,4,4a,5,6,7,8,8a-octahydronaphthalene(70:30) was used. Polymer (C) had a weight average molecular weight (Mw)of 36,000, and contained 0.5% by weight of a fraction with a molecularweight of not larger than 1,000. The hydrogenation percentage of polymer(B) was 99.9%. The glass transition temperature was 110° C. The contentof repeating units having an alicyclic structure with two rings havingno norbornane structure was 70% by weight.

REFERENCE EXAMPLE 4

[0173] A reactor was thoroughly dried and flashed with nitrogen, and wascharged with 25 parts of tetracyclododecene (TCD), 25 parts ofdicyclopentadiene (DCP), 1% by mole, based on the total monomers, of1-hexene, and 300 parts of toluene. Then 16 parts of a solution with a 1mole concentration of triethylaluminum in toluene, 4 parts oftriethylamine, and 3 parts of a solution with a 1 mole concentration oftitanium tetrachloride in toluene were added to effect polymerization at25° C. for 2 hours.

[0174] The reaction mixture was put into a mixture of acetone/isopropylalcohol (1:1) to coagulate a polymer, and the precipitate was filteredand dried to give 74 parts of a polymer. The yield was 74%. Analysis ofthe polymer by proton NMR spectrum revealed that the polymer containedthe TCD ingredient and the DCP ingredient at a weight ratio of 25:75.Then 50 parts of the polymer was dissolved in 500 parts of cyclohexane,and was subjected to hydrogenation reaction at a temperature of 140° C.for 4 hours under a hydrogen pressure of 60 kg/cm² by using 5 parts of apalladium/carbon catalyst. The obtained polymer solution was filtered toremove the catalyst, and then, put into a mixed solvent ofacetone/isopropyl alcohol (1:1) to coagulate the polymer. Thethus-obtained precipitate was filtered and dried to give 43 parts of apolymer (D).

[0175] Polymer (D) had a weight average molecular weight (Mw) of 68,000,and contained 0.6% by weight of a fraction with a molecular weight ofnot larger than 1,000. The hydrogenation percentage of polymer (D) was99.9%. The glass transition temperature was 127° C. The content ofrepeating units having an alicyclic structure with two rings having nonorbornane structure was 75% by weight.

REFERENCE EXAMPLE 5

[0176] An autoclave having an inner volume of 1 liter was charged with100 g of 8-methyl-8-methoxycarbonyl-tetracyclo-[4.4.0.1^(2,5).1^(7,8)]dodeca-3-ene, 60 g of 1,2-dimethoxyethane, 240 g of cyclohexane,9 g of 1-hexene and 3.4 ml of a solution with a concentration of 0.96mole/l of diethylaluminum chloride in toluene.

[0177] A flask was charged with 20 ml of a solution with a concentrationof 0.05 mole/l of tungsten hexachloride in 1,2-dimethoxymethane and 10ml with a concentration of 0.1 mole/l of para-aldehyde in1,2-dimethoxyethane. Then 4.9 ml of the thus-obtained mixed solution wasincorporated into the above-mentioned content of the autoclave. Theautoclave was closed and the mixture was heated at 80° C. for 2.5 hourswith stirring.

[0178] To the thus-obtained polymer solution, a mixed solvent of1,2-dimethoxyethane/cyclohexane (2/8) was added to prepare a polymersolution (polymer/solvent ratio=1/9 by weight), and further 20 g oftriethanolamine was added and stirred for 10 minutes. Then 500 g ofmethanol was added to the polymer solution, and the mixture was stirredfor 30 minutes and allowed to stand to be thereby separated into twolayers. The upper layer was removed, and methanol was again added to thelower layer. The mixture was stirred and allowed to stand to be therebyseparated into two layers, followed by removal of the upper layer. Thisoperation was repeated further two times, and the obtained lower layerwas diluted with cyclohexane and 1,2-dimethoxyethane to give a polymersolution with a concentration of 10% in cyclohexane-1,2-dimethoxyethane.20 g of palladium/silica-magnesia [supplied by Nikki Kagaku K.K.,palladium content: 5%] was added to the polymer solution andhydrogenation was conducted under a hydrogen pressure of 40 kg/cm²at165° C. for 4 hours. The hydrogenation catalyst was removed byfiltration to give a solution of a hydrogenated polymer.

[0179] To 100 parts of the solution of the hydrogenated polymer, 0.1part ofpentaerythrityl-tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]as antioxidant was added, and the polymer solution was dried at 380° C.under a reduced pressure to remove the solvent. Then the molten polymerwas pelletized by using an extruder in a nitrogen atmosphere. Thethus-obtained polymer (E) had an intrinsic viscosity of 0.48 dl/g (30°C., in chloroform). The hydrogenation percentage was 99.5%. The glasstransition temperature was 168° C. The content of repeating units havingan alicyclic structure with two rings having no norbornane structure was0% by weight.

EXAMPLES 1 TO 3

[0180] Making of Flat Plate

[0181] To 100 parts of each of polymers (A), (B) and (C) prepared inReference Examples 1 to 3, 0.1 part ofpentaerythrityl-tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]as antioxidant was added, and the mixture was kneaded together at 250°C. and pelletized by using an extruder. The pellet was injection-moldedat a polymer temperature of 280° C. by using a commercially availableinjection molding machine to give a flat plate having a size of 15 cm×15cm×1 mm. The flat plates exhibited a high light transmittance.Mechanical strength, Δ YI and distortion due to moisture absorption wereevaluated. The results are shown in Table 1.

[0182] Making of Light Guide Plate

[0183] The above-mentioned pellet was extrusion-shaped into a lightguide plate with a wedge-shaped cross-section shown in FIG. 1 and havingthe following sizes.

[0184] Diagonal length: 11 inch

[0185] Thickness at incident side end: 3 mm

[0186] Thickness at opposite side end: 0.8 mm

[0187] Depth of V-shaped groove: 5 μm

[0188] Top angle α of V-shaped groove: 140°

[0189] Largest pitch at incident side end of grooves: 1 mm

[0190] Smallest pitch at opposite side end of grooves: 0.2 mm

[0191] Using the light guide plate, a back light unit as illustrated inFIG. 1 was assembled. Light was rendered incident at a lamp current of 5mA on the incident side end, and luminance on the illuminating surfacewas measured. The measurement of luminance was conducted on nine pointsuniformly distributed on the effective emitting region surrounded by theperipheral edge area with a width of 10 mm on the illuminating surface.The light guide plates made from polymers (A), (B) and (C) exhibited anaverage luminance of larger than 1,500 cd/m² and a fluctuation inluminance of below 20%.

COMPARATIVE EXAMPLE 1

[0192] A flat plate was injection-molded from polymer (D) prepared inReference Example 4, by the same procedure as employed in Example 1wherein the polymer temperature was changed to 350° C. with all otherconditions remaining the same. Mechanical strength, ΔYI and distortiondue to moisture absorption were evaluated. The results are shown inTable 1.

COMPARATIVE EXAMPLE 2

[0193] A flat plate was injection-molded from polymer (E) prepared inReference Example 5, by the same procedure as employed in Example 1wherein the polymer temperature was changed to 320° C. with all otherconditions remaining the same. Mechanical strength, ΔYI and distortiondue to moisture absorption were evaluated. The results are shown inTable 1. TABLE 1 Crack occurrence Phase Polymer at weight at moldingRmax difference Emission point No. materials drop (%) (%) ΔYIDistortion** (μm) (nm) and dark point Ex. 1 A a 0 a 0 a a 0.1 23 noneEx. 2 B a 0 b 10 a a 0.3 28 none Ex. 3 C b 15 b 13 a a 1.1 19 *1 Com. 1D b 26 b 22 d a 1.5 60 *2 Com. 2 E d 100 c 55 d b 2.5 75 *3

[0194] As seen from Table 1, the flat plates of the invention have highmechnical strength, resistance to discoloration and resistance todistortion due to moisture absorption (Example 1 to 3). In the casewhere the cycloolefin polymer used has a salient proportion of repeatingunits having no nornbornane structure, the flat plate exhibits excellentmechanical strength (at a weight-dropping test) (Examples 1, 2>Example3>Comparative Example 1). In the case where the cycloolefin polymer usedcontains only very minor amount of a low-molecular-weight fraction, themechanical strength is more enhanced (comparison of Example 1 withExample 2). When the cycloolefin polymer has repeating units consistingof a norbornane structure, the mechanical strength, resistance todiscoloration and distortion due to moisture absorption are relativelypoor (Comparative Example 2). When the molecular weight of thecycloolefin polymer is too large, the resistance to discoloration ispoor (Comparative Example 1).

What is claimed is:
 1. A flat plate made of a cycloolefin polymercomprising at least 30% by weight, based on the cycloolefin polymer, ofrepeating units (A) having an alicyclic structure; said repeating units(A) comprising at least 30% by weight, based on the repeating units (A),of repeating units (A-i) having no norbornane structure and more than 0%hut not more than 70% by weight of repeating units (A-ii) having anorbornane structure, represented by the following formula (4):

wherein R³⁵ through R⁴⁸ are independently selected from the groupconsisting of a hydrogen atom, hydrocarbon, hydroxyl, ester, alkoxy,cyano, imido and silyl groups, a halogen atom, and hydrocarbon groupshaving a functional group as substituent selected from the groupconsisting of hydroxyl, ester, alkoxy, cyano, imido and silyl groups;R³⁸ and R³⁹, or R⁴⁰ and R⁴¹ may form together an alkylidene group having1 to 20 carbon atoms; e is an integer of 1 to 3, and

represents a carbon-carbon single or double bond; and said cycloolefinpolymer having a weight average molecular weight of 5,000 to 50,000,wherein an illuminating surface of the flat plate has a surfaceroughness of not larger than 5 μm as expressed in terms of the maximumheight (Rmax).
 2. The flat plate according to claim 1, wherein therepeating units (A-i) having no norbornane structure are represented bythe formula (2):

R²¹ through R³⁴ are independently selected from the group consisting ofa hydrogen atom, hydrocarbon, hydroxyl, ester, alkoxy, cyano, imido andsilyl groups, a halogen atom, and a hydrocarbon group having afunctional group as substituent selected from the group consisting ofhydroxyl, ester, alkoxy, cyano, imido and silyl groups, R²⁵ and R²⁷, orR²⁶ and R²⁸ may form together an unsaturated bond; or R²⁵and R²⁶, or R²⁷and R²⁸ may form together an alkylidene group; d is 0 or 1; and

represents a carbon-carbon single or double bond.
 3. The flat plateaccording to claim 1, wherein the repeating units (A-i) having nonorbornane structure are represented by the formula (3):


4. The flat plate according to claim 1, wherein the repeating units(A-ii) having a norbornane structure are represented by the formula (5):

wherein R⁴⁹ through R⁶² are the same as R³⁵ through R⁴⁸ as defined informula (4), respectively; R⁵² and R⁵³, or R⁵⁴ and R⁵⁵ may form togetheran alkylidene group, and

represents a carbon-carbon single or double bond.
 5. The flat plateaccording to claim 1, wherein the repeating units (A-ii) having anorbornane structure are represented by the formula (6):

wherein R⁶³ through R⁶⁶ are the same as R³⁸ through R⁴¹ as defined informula (4), respectively, and

represents a carbon-carbon single or double bond.
 6. The flat plateaccording to claim 1, wherein the cycloolefin polymer has a temperatureof at least 280° C. at which the loss of polymer on heating reaches 5%by weight.
 7. The flat plate according to claim 1, wherein thecycloolefin polymer has a melt viscosity of 1×10² to 1×10⁵ poise asmeasured at a temperature of 260° C.
 8. The flat plate according toclaim 1, wherein the cycloolefin polymer contains not larger than 20% byweight, based on the cycloolefin polymer, of a fraction having amolecular weight of not larger than 1,000.
 9. The flat plate accordingto claim 1, wherein the cycloolefin polymer contains 0.2% to 20% byweight, based on the cycloolefin polymer, of a fraction having amolecular weight of not larger than 1,000.
 10. The flat plate accordingto claim 1, wherein the phase difference of the flat plate as measuredin the direction of thickness is not larger than 100 nm.
 11. The flatplate according to claim 1, wherein the repeating units (A) having analicyclic structure comprises, based on the repeating units (A), atleast 50% by weight of repeating units (A-i) having no norbornanestructure and more than 0% but not more than 50% by weight of repeatingunits (A-ii) having a norbornane structure.
 12. The flat plate accordingto claim 1, wherein the repeating units (A) having an alicyclicstructure comprises, based on the repeating units (A), at least 70% byweight of repeating units (A-i) having no norbornane structure and morethan 0% but not more than 30by weight of repeating units (A-ii) having anorbornane structure.
 13. A light guide plate composed of the flat plateas claimed in claim
 1. 14. The light guide plate according to claim 13,which has a wedge-shaped cross-section.
 15. The light guide plateaccording to claim 13, which has a light reflection plane with a printedpattern.
 16. The light guide plate according to claim 13, which has alight reflection plane with a plurality of V-shaped grooves.
 17. Thelight guide plate according to claim 13, which has a wedge-shapedcross-section and a light reflection plane with a plurality of V-shapedgrooves which have pitches gradually decreasing from the light-incidentside end toward the tip end of the wedge-shaped cross-section.
 18. Thelight guide plate according to claim 13, which has an illuminatingsurface with a surface area of 10 to 2,500 cm² and has a thickness of0.5 to 10 mm.
 19. The light guide plate according to claim 13, whereinthe phase difference of the light guide plate as measured in thedirection of thickness is not larger than 100 nm.